1. Field of the Invention
The invention pertains to an adhesive application process in book manufacturing during the joining of the book block and the cover using a PUR melt adhesive system (PUR=polyurethane) that undergoes cross-linking in the presence of moisture. The term "book manufacture" in the present connection, aside from the manufacturing of books in the true sense, is also defined to include the manufacture of other graphic products, such as journals and catalogs. The terms "book block", "book spine" and the like used in the following have a corresponding general significance.
2. Statement of Related Art
For the adhesive binding of graphic products on table-type devices as well as semiautomatic or completely automatic adhesive binding machines, the adhesive is applied via roller application systems, directly to the book spine. For this purpose, open glue buckets are used. Attempts have already been made to likewise apply the adhesive directly to the spine of the book using nozzles adapted to the shape of the spine. However, this procedure failed due to the fact that the marginal leaves on the cut edge were unable to be reliably or completely covered with adhesive. Especially in the case of high-volume papers, in which the degree of deformation is particularly high as a result of the pressure of the grippers required to hold the book, problems often occur.
In practice, dispersion adhesives, glutin adhesives and melt adhesives are used for adhesion. However, the application range of these adhesive groups is limited, since the papers which are difficult to join depending on the adhesive type are no longer bonded with a strength sufficient for the stress of utilization. For example, volatile components of printing ink that may have been applied to the contents page or even the ink itself or constituents of the surface coating from the cover may migrate into the adhesive and reduce its strength. In addition, there is limited flexibility at low temperatures of less than -20.degree. C. as well as a decrease in strength at higher temperatures of more than 80.degree. C.
The problems in the systems mentioned, operating with single-layer or multi-layer application, can be largely eliminated in and of themselves by using single-component or two-component PUR melt adhesives. However, whereas two-component systems can still be processed and mastered well with the above-mentioned roller application systems, this is no longer true for the PUR melt adhesive systems which undergo cross-linking in the presence of moisture. To be sure, an improvement can be achieved by means of substantially smaller, largely encapsulated application systems, but the fundamental risk of undesired cross-linking remains. In the case of longer work intervals, which can occur in the case of format changes or machine change-over, or in the case of problems occurring with machines located earlier in the process stream, the application system must therefore be cleaned, or the risk of cross-linking of the adhesive still present in the application system must be taken into consideration. Since completely cross-linked PUR melt adhesives are not soluble or fusible, cleaning can only be accomplished mechanically, with a correspondingly high expenditure of time.
From package gluing, technology adhesive application systems are already known from German Published Application No. 1,274,864 and German Preliminary Published Application No. 1,922,056 which, equipped with one or several nozzles, can continuously or discontinuously apply adhesive in a manner limited in terms of width and length to a substrate, for example the closing strip of a cardboard carton. Dispersion adhesives as well as animal glues and melt adhesives are suitable for this application system.